Control of liquid level of refrigerants in serially connected indirect heat exchangers



April 7, 1970 w HOBBS ET AL 3,504,735

CONTROL OF LIQUID LEVEL'OF REFRIGERANTS IN SERIALLY CONNECTED INDIRECTHEAT EXCHANGERS Filed NOV. 19, 1968 A W HHH 3 u R 0 W 1 EM m mm 0 @Q $13 m wzmjzim J a l 3 mww w w m 8 u mm 0 A W m mm 5 mm 5 mm 1 M24252 mm x3 w mm a 6 mm v m mw mv a Q. 3 5 1 MZ IEE+ mm mm mm I United StatesPatent 3,504,735 CONTROL OF LIQUID LEVEL OF REFRIGERANTS IN SERIALLYCONNECTED INDIRECT HEAT EXCHANGERS James W. Hobbs, Bartlesville, Okla.,and Dale E. Lupfer,

Sweeny, Tex., assignors to Phillips Petroleum Company, a corporation ofDelaware Filed Nov. 19, 1968, Ser. No. 776,976 Int. Cl. F25j 3/08 US.Cl. 1651 8 Claims ABSTRACT OF THE DISCLOSURE A feedstream to be cooledis passed through first and second indirect heat exchangers in series. Arefrigerant is passed at a variable flow rate from a limited sourcethereof through the second heat exchanger. The flow rate of arefrigerant to the first heat exchanger is manipulated to vary theliquid level of refrigerant in the first heat exchanger responsive tothe liquid level of refrigerant in the second heat exchanger.

The invention relates to method and apparatus for controlling thecooling of a feedstream by indirect heat exchange with two refrigerants,one of the refrigerants being variable in the quantity available.

In the past, it has been common to vary the flow rate of an at leastpartially liquefied refrigerant to an indirect heat exchanger responsiveto the liquid level of the refrigerant in the heat exchanger. However,such control is unsatisfactory when employing a refrigerant of varyingand limited availability. Cooling control is lost when the required heattransfer exceeds the cooling power of the available refrigerant, andcooling power is wasted when the heat transfer requirement is less thanthe cooling power of the available refrigerant.

In accordance with the invention, it has been discovered that thecooling power of the refrigerant of limited availability can be employedto the fullest while maintaining the desired control of the cooling ofthe feedstream by passing the feedstream to be cooled through twoindirect heat exchangers in series, with the refrigerant of limitedavailability being utilized in the second exchanger, and varying theliquid level of another refrigerant in the first exchanger responsive tothe liquid level of the refrigerant of limited availability in thesecond exchanger.

Accordingly, it is an object of the invention to provide improved methodand apparatus for controlling the cooling of a feedstream. Anotherobject is to utilize the cooling power of a refrigerant from a variablelimited source to the maximum extent possible. Another object of theinvention is to maintain the desired heat transfer while employing arefrigerant having a variable flow rate from a limited source. Otherobjects, aspects and advantages of the invention will be a parent from astudy of the specification, the drawing, and the appended claims to theinvention.

The drawing is a diagrammatic representation of a process incorporatingthe control system of the present invention.

Referring now to the drawing in detail, a feedstream comprisinghydrogen, methane, ethylene, ethane, and propylene is passed throughconduit 1 into and through coil 2 in indirect heat exchanger 3, whereinthe feedstream is cooled by heat exchange with a vaporizing refrigerant,for example, propylene. The at least partially liquid refrigerant isintroduced into the shell of exchanger by way of conduit 4. Theresulting refrigerant vapors are withdrawn from exchanger 3 by way ofconduit 5. A valve 6, positioned in conduit 4, can be manipulated byliquid level controller 7 responsive to a comparison of the actual level"ice of liquid refrigerant in the shell of exchanger 3 with the desiredliquid level represented by setpoint 8. Valve 9, positioned in conduit5, is manipulated by pressure recorder controller 10 responsive to acomparison of the refrigerant vapor pressure in the shell of exchanger 3as indicated by pressure sensor 11 and the desired vapor pressurerepresented by setpoint 12. The thus cooled feedstream is withdrawn fromexchanger 3 and passed by way of conduit 14 into and through coil 15 ofindirect heat exchanger 16. An at least partially liquid stream ofethane is passed by way of conduit 17 into the shell of exchanger 16,wherein the ethane is vaporized to further cool the feedstream in coil15. The resulting vaporized ethane is withdrawn from the shell ofexchanger 16 by way of conduit 18 and passed to further processing,utilization or storage. A valve 19, positioned in conduit 18, can bemanipulated by pressure recorder controller 21 responsive to acomparison of the output signal from pressure sensor 22 which isrepresentative of the actual refrigerant vapor pressure in the shell ofexchanger 16 and a setpoint signal 23 which is representative of thedesired refrigerant vapor pressure in the shell of exchanger 16. Thefurther cooled feedstream is withdrawn from coil 15 and passed by way ofconduit 26 into and through coil 27 of indirect heat exchanger 28. Arefrigerant, for example ethylene, is passed through conduit 29 into theshell of exchanger 28 and is withdrawn therefrom by way of conduit 31.The still further cooled feedstream, which is partially liquefied, isthen passed through conduit 32 into liquid-vapor separation tank 33.

A liquid stream, comprising ethylene, ethane and propylene, is withdrawnfrom the bottom of tank 33 by way of conduit 34 and passed to furtherprocessing, utilization or storage. A valve 35, located in conduit 34,can be manipulated by liquid level controller 36 to maintain the actualliquid level in tank 33 substantially at a desired value represented bysetpoint 37. A vapor stream, comprising hydrogen, methane, ethylene andethane, is withdrawn from an upper portion of tank 33 and passed by wayof conduit 38, coil 39 of indirect heat exchanger 41 and conduit 42 intoliquid vapor separator 43. A refrigerant, for example ethylene, ispassed through conduit 44 into exchanger 41 to partially liquefy thefluid in coil 39. The resulting warmed refrigerant is withdrawn from eX-changer 41 by Way of conduit 45. A valve 46, located in conduit 38, canbe manipulated by pressure recorder controller 47 responsive to acomparison of the actual pressure in tank 33 as indicated by pressuresensor 48 and the desired pressure represented by setpoint 49.

A vapor stream, comprising primarily hydrogen and methane, is withdrawnfrom an upper portion of separator 43 by way of conduit 51. A valve 52,positioned in conduit 51, can be manipulated by pressure recordercontroller 53 responsive to a comparison of the actual pressure inseparator 43 as indicated by pressure sensor 54 and the desired pressurerepresented by setpoint 55. A liquid stream, comprising primarilymethane, ethylene and ethane, is withdrawn from a lower portion ofseparator 43 and passed by way of conduit 56 into demethanizingfractionator 57. A valve 58 positioned in conduit 56 can be manipulatedby liquid level controller 59 to maintain a liquid level in separator 43substantially at the value represented by setpoint 61.

A vapor stream, comprising primarily methane, is withdrawn from an upperportion of fractionator 57 through conduit 62. A liquid stream,comprising ethylene and ethane, is passed from a lower portion offractionator 57 through conduit 63 into fractionator 64. A valve 65,positioned in conduit 63, can be manipulated by liquid level controller66 to maintain a liquid level in fractionator 57 at a value representedby setpoint 67. A vaporous ethylene stream is withdrawn from an upperportion of fractionator 64 by way of conduit 68 while a liquid ethanestream is withdrawn from a lower portion of fractionator by way ofconduit 17. Liquid level controller 71 manipulates valve 72 in conduit17 responsive to setpoint 73 to maintain a desired liquid level infractionator 64.

In accordance with the present invention, liquid level controller 74compares a signal representative of the liquid ethane level in the shellof exchanger 16 with a setpoint signal 75 representative of the desiredliquid ethane level in the shell of exchanger 16 and produces an outputsignal representative of the difference between the two signals. Thisoutput signal is applied to the setpoint input 8 of controller 7. Thispermits the efiicient utilization of all of the ethane available fromthe kettle product of fractionator 64, which varies in amount asdictated by controller 71. Moreover, the control system of the inventionprovides the desired cooling of the feedstream. The temperature of theethane in the shell of exchanger 16 is maintained constant at theboiling point of ethane corresponding to the pressure represented by thesetpoint 23 to pressure controller 21. As the heat transfer requirementincreases due to the temperature of the feedstream in conduit 14 risingand/ or the flow rate of the feedstream through conduit 14 increasing,the liquid ethane in the shell of exchanger 16 is vaporized at a fasterrate, causing a small drop in the liquid ethane level in exchanger 16below the level represented by setpoint 75. This results in an increasein the output signal from controller 74 which is applied to the setpointinput of level controller 8, thereby calling for a higher liquidrefrigerant level in exchanger 3. Controller 7 increases the opening ofvalve 6 to pass refrigerant through conduit 4 into the shell ofexchanger 3 at a higher rate. As the liquid level rises, the heattransfer rate increases, thereby providing additional cooling to thefeedstream. Similarly, a decrease in the heat transfer requirements forexchanger 16 causes a slight rise in liquid ethane level in exchanger16, a decrease in the setpoint signal to controller 7, and a partialclosing of valve 6. Thus, the control system provides for the maximumutilization of the cooling power of a variable flow rate refrigerantstream from a limited source thereof while maintaining the desiredcooling of the feedstream.

While exchangers 3, 16 28 and 41 can be shell and tube heat exchangers,one or more can be replaced with any other type of indirect heatexchanger having at least two flow paths therethrough with the liquidlevel in one of the flow paths being variable to vary the heat transferrate. Pressure reduction valve can be employed in any of conduits 12,26, 32, 42, 56 and 63 to increase the autorefrigeration of thefeedstream. While the invention has been illustrated with all of thefeedstream being passed from coil 2 to coil 15, part of the feedstreamcan be withdrawn from conduit 14 or additional feed can be added toconduit 14.

We claim:

1. Apparatus comprising a first indirect heat exchanger having first andsecond flow paths therethrough,

a second indirect heat exchanger having first and second fiow pathstherethrough,

first conduit means for passing a feedstream to be cooled to an inlet ofsaid first flow path of said first indirect heat exchanger, secondconduit means for passing at least a portion of said feedstream from anoutlet of said first flow path of said first indirect heat exchanger toan inlet of said first flow path of said second indirect heat exchanger,

third conduit means for passing an at least partially liquefiedrefrigerant to an inlet of said second flow path of said first indirectheat exchanger,

fourth conduit means for withdrawing vaporized refrigerant from anoutlet of said second flow path of said first indirect heat exchanger,

fifth conduit means for passing an at least partially liquefiedrefrigerant stream at the available flow rate from a limited sourcethereof to an inlet of said second flow path of said second indirectheat exchanger,

sixth conduit means for withdrawing vaporized refrigerant from an outletof said second flow path of said second indirect heat exchanger,

means for varying the liquid level of refrigerant in said second flowpath of said first indirect heat exchanger to maintain the liquid levelof refrigerant in said second flow path of said second indirect heatexchanger substantially constant, and

seventh conduit means for withdrawing the thus cooled feedstream from anoutlet of said first fiow path of said second indirect heat exchanger.

2. Apparatus in accordance with claim 1 further comprising means formaintaining the pressure of refrigerant in said second flow path of saidfirst indirect heat exchanger substantially constant, and means formaintaining the pressure of refrigerant in said second flow path of saidsecond indirect heat exchanger substantially constant.

3. Apparatus in accordance with claim 1 wherein said means for varyingthe liquid level comprises means for measuring the actual liquid levelof refrigerant in said second flow path of said second indirect heatexchanger and establishing a first signal representative thereof, meansfor establishing a second signal representative of the desired liquidlevel of refrigerant in said second fiow path of said second indirectheat exchanger,

means responsive to said first and second signals to establish a thirdsignal representative of the difference between said first and secondsignals,

means for measuring the actual liquid level of refrigerant in saidsecond flow path of said first indirect heat exchanger and establishinga fourth signal representative thereof, means responsive to said thirdand fourth signals for establishing a fifth signal representative of thedifference between said third and fourth signals, and

means for varying the flow rate of refrigerant through said thirdconduit means responsive to said fifth signal.

4. Apparatus in accordance with claim 3 further comprising means forvarying the rate of withdrawal of refrigerant vapors from said secondflow path of said second indirect heat exchanger to maintain thepressure in said second flow path of said second indirect heat exchangersubstantially constant, and means for varying the rate of withdrawal ofrefrigerant vapors from said second flow path of said first indirectheat exchanger to main tain the pressure in said second flow path ofsaid first indirect heat exchanger substantially constant.

5. Apparatus in accordance with claim 1 further comprising means forseparating the feedstream passing through said seventh conduit meansinto at least two fractions, and means for passing one of said fractionsto said fifth conduit means as the source of refrigerant for said fifthconduit means.

6. A method of cooling at feedstream which comprises passing saidfeedstream through a first flow path of a first indirect heat exchangingzone and then through a first flow path of a second indirect heatexchanging zone,

passing an at least partially liquefied first refrigerant through asecond flow path of said first indirect heat exchanging zone in indirectheat exchanging relationship With the feedstream in said first flow pathof said first indirect heat exchanging zone to cool said feedstream byvaporizing said first refrigerant,

passing an at least partially liquefied second refrigerant at a variableflow rate from a limited source thereof through a second flow path ofsaid second indirect heat exchanging zone in indirect heat exchangingrelationship with the feedstream in said first flow path of said secondindirect heat exchanging zone to cool said feedstream by vaporizing saidsecond refrigerant, and varying the liquid level of said firstrefrigerant in said second flow path of said first indirect heatexchanging zone to maintain the liquid level of said second refrigerantin said second flow path of said second indirect heat exchanging zonesubstantially constant. 7. A method in accordance with claim 6 furthercomprising maintaining the pressure of said first refrigerant in saidsecond flow path of said first indirect heat exchanging zonesubstantially constant, and maintaining the pressure of said secondrefrigerant in said second flow path of said second indirect heatexchanging zone substantially constant.

8. A method in accordance with claim 7 further comprising withdrawingthe thus cooled feedstream from said 6 first flow path of said secondindirect heat exchanging zone, separating said thus cooled feedstreaminto at least two fractions, and utilizing one of said fractions as saidsecond refrigerant.

References Cited UNITED STATES PATENTS 3,255,596 6/1966 Greco et a1.6221 MEYER PERLIN, Primary Examiner C. SUKALO, Assistant Examiner US.Cl. X.R.

